Thermospheric wind response to a sudden ionospheric variation in the trough: event at a pseudo-breakup during geomagnetically quiet conditions

<jats:title>Abstract</jats:title><jats:p>The thermospheric wind response to a sudden westward turning of the ion velocity at a high latitude was studied by analyzing data obtained with a Fabry–Perot interferometer (FPI; 630 nm), Dynasonde, and Swarm A & C satellites during a conjunction event. The event occurred during a geomagnetically quiet period (Kp = 0 +) through the night, but some auroral activity occurred in the north. The collocated FPI and Dynasonde measured the thermospheric wind (<jats:bold>U</jats:bold>) and ionospheric plasma velocity (<jats:bold>V</jats:bold>), respectively, in the F region at the equatorward trough edge. A notable scientific message from this study is the possible role of thermospheric wind in the energy dissipation process at F-region altitude. The FPI thermospheric wind did not instantly follow a sudden <jats:bold>V</jats:bold> change due to thermospheric inertia in the F region. At a pseudo-breakup during the event, <jats:bold>V</jats:bold> suddenly changed direction from eastward to westward within 10 min. <jats:bold>U</jats:bold> was concurrently accelerated westward, but its development was more gradual than that of <jats:bold>V</jats:bold>, with <jats:bold>U</jats:bold> remaining eastward for a while after the pseudo-breakup. The delay of <jats:bold>U</jats:bold> is attributed to the thermospheric inertia. During this transition interval, <jats:bold>U</jats:bold>∙<jats:bold>V</jats:bold> was negative, which would result in more efficient generation of frictional heating than the positive <jats:bold>U</jats:bold>∙<jats:bold>V</jats:bold> case. The sign of <jats:bold>U</jats:bold>∙<jats:bold>V</jats:bold>, which is related to the relative directions of the neutral wind and plasma drift, is important because of its direct impact on ion-neutral energy exchange during collisions. This becomes especially important during substorm events, where rapid plasma velocity changes are common. The sign of <jats:bold>U</jats:bold>∙<jats:bold>V</jats:bold> may be used as an indicator to find the times and locations where thermospheric inertia plays a role in the energy dissipation process.</jats:p> <jats:p><jats:bold>Graphical Abstract</jats:bold></jats:p>